In oil and gas wells drilled throughout the world, a well bore liner, or casing is placed inside the open wellbore to maintain wellbore stability and to control formation pressures. In many of these wells, an intermediate casing is installed and cemented into place. After the intermediate casing is installed, drilling can be continued through this casing to a deeper depth. Formations outside the intermediate casing are thus isolated, which helps to eliminate well bore cave in and to contain formation pressures and fluids. The cementing of the intermediate casing to the formation is critical to obtain these goals. If the cement bond is poor, formation pressures can migrate through the inadequate cement to the surface, resulting in an uncontrollable well bore.
To cement the casing into drilled well bore, a pre-calculated volume of cement is pumped down the inside of the casing and up the outside of the casing, to surface. Once the cement is pumped inside the casing, a drillable rubber wiper plug or wiper plug, is installed inside the casing on top of the cement slurry. Water is then pumped inside the casing to push the wiper plug downwards and thereby displace the cement slurry from the inside of the casing to the outside of the casing. Once the casing volume is filled with water and the wiper plug has reached the bottom of the casing, the cement fills the cavity between the casing and the drilled well bore and reaches the surface. Pumping pressures will increase when the plug reaches the bottom of the casing, informing personnel that the cement is in place and the plug has reached the bottom of the casing. Once adequate time has expired for cement curing, the plug can be drilled out and drilling to deeper depths can continue.
In order to rotate the casing or change casing elevations while cementing, a cementing attachment, called a cement head is placed at the top of the casing. The cement head contains hoses and piping to transfer the cement and water from pumps to the inside of the casing. It also contains valves to launch the wiper plug down inside the casing and bearing assemblies to isolate the movement of the hoses and piping in order to rotate the casing.
Most cement heads are cumbersome and only some offer rotational movement of the casing while most will allow only vertical movement, but never both reciprocation and rotation. While cementing under high pressures, piping connections on cement heads may vibrate loose and the cementing job must be temporarily stopped to tighten fittings. Hoses and piping must be reasonably lengthy in order to provide sufficient allowance for the casing to be reciprocated up and down within the rig derrick, causing hazards on the rig floor.
The typical cement head has two valves located above and below the wiper plug, each connected to a manifold and then to one common pumping line. The cement line and cement valve are located below the wiper plug, while the water line and water valve are located above the wiper plug. While pumping the cement slurry, the cement valve is open and the water valve is closed so that cement bypasses the plug to place cement inside the casing. Once the proper volume of cement is pumped, the cement valve is then closed and the water valve is opened. The pumped water above the plug, launches the plug and continues to displace the volume of cement.
While most rigs have top drives, rotation of the casing can be achieved by connecting the drive to the top of the cement head. Those rigs that rotate using a rotary table are unable to rotate and reciprocate the casing at the same time, unlike the top drive rig. Since most cement heads have lines attached to them, rotation is impossible without the use of bearings to keep the lines stationary. With the use of these bearings, rotation of the casing is not achievable with the top drive, but only with the rotary table. When the rotary table is being used, reciprocate of the casing is not possible.
On some vertically drilled wells and on most horizontally drilled wells, the casing does not stay centered within the drilled well bore while placing cement slurries. On horizontal wells, the casing does not hang pendulum to the well bore like a vertical well, and stabilization often cannot withstand the weight of the casing from contacting the horizontal well bore. In these situations, the cement will channel through the larger cavity between the casing and well bore, placing cement in only some areas of the wellbore cavity. In many cases, lack of movement of the casing causes the casing to stick to the wellbore walls before or during the cementing operation. Allowing the casing to be rotated and reciprocated at the same time will allow maximum cement coverage around the casing and will avoid channeling.
Furthermore, there is a need for a cement head tool that is simple and quick to load or pre-load and assemble. Importantly, there is a need to be able to pre-load and pre-assemble a cement head to provide a quick transition in the field to cementing operations.